A neologistic derogatory term originally coined in the Midlands of the UK. It is a compound of the words cunt and 'by', a reference to Mitchell and Webb's (British comedians) recurring sketch character Sir Digby.
It is traditionally used in self-effacing situations (see example 1), but can be used as a general insult (example 2).
1. "I'm SUCH a cuntby!"
2. "Yeah I heard him say that... What a cuntby."